Wednesday, November 14, 2012

A brief note on http pipeline status - you may have read on mozilla planet yesterday that Chrome has this technology enabled. That's not true, I can't say why the author wrote it - but chrome has the same status (implemented but configured off by default) as firefox and largely for the same reasons.

This is a painful note for me to write, because I'm a fan of HTTP pipelines.

But, despite some obvious wins with pipelining it remains disabled as a high risk / high maintenance item. I use it and test with it every day with success, but much of the risk in tied up in a user's specific topology - intermediaries (including virus checkers - an oft ignored but very common intermediary) are generally the problem with 100% interop. I encourage readers of planet moz to test it too - but that's a world apart turning it on by default- the test matrix of topologies is just very different.

See bugzilla 716840 and 791545 for some current problems caused by unknown forces.

Also see telemetry for TRANSACTION_WAIT_TIME_HTTP and TRANSACTON_WAIT_TIME_HTTP_PIPELINES - you'll see that pipelines do marginally reduce queuing time, but not by a heck of a lot in practice. (~65% of transactions are sent within 50ms using straight HTTP, ~75% with pipelining enabled). If we don't see more potential than that, it isn't worth taking the pain of interop troubles. It is certainly possible that those numbers can be improved.

The real road forward though is HTTP/2 - a fully multiplexed and pipelined protocol without all the caveats and interop problems. Its better to invest in that. Check out TRANSACTON_WAIT_TIME_SPDY and you'll see that 93% of all transactions wait less than 1ms in the queue!